Insect - Fungal - Bacterial & Viral

Insects

It is commonly assumed that hydroponic agriculture systems are relatively free of insect pests and plant diseases because the technology is mostly enclosed. Unfortunately, this is not true. Pest populations can increase with alarming speed in greenhouses because of the lack of natural environmental checks.

The frightening ability of some insects to develop resistance to pesticides has revived worldwide interest in the concept of biological control: the deliberate introduction of natural enemies of pest insects, particularly when used in association with horticultural practices, plant genetics and other central mechanisms.

While there are many pests and diseases which attack tomatoes, below is a list of a few of the major pests associated with hydroponic tomato production and their control.

 

Whiteflies

There are about 1,200 different species of whiteflies. They are pests in many important agricultural and horticultural crops, both inside and outside the greenhouse environment.

Trialeurodes vaporariorum

The greenhouse whitefly (Trialeurodes vaporariorum) has been a problem for greenhouse tomatoes in the U.S. since 1870. Originally from tropical or subtropical America , probably Brazil or Mexico.

Life cycle of the greenhouse whitefly:

 

Photo by Oscar Minkenberg

Length of the life cycle depends on temperature, ranging from 4 weeks at 27 C to over 8 weeks at 14 C degrees. Damage is caused by flies and larvae sucking the leaf sap, which can cause stunting, leaf drop, and reduced yield. Honeydew deposits on fruit are sticky and can mold, making the fruit unmarketable. Greenhouse whitefly can transmit viruses. And although whiteflies cannot hibernate, the eggs of the greenhouse whitefly can survive for about 5 days at temperatures of -6 C.

Bemisia tabaci

Tobacco whitefly, Bemisia tabaci, also known as the sweetpotato, silverleaf, or cotton whitefly, first occurred in Greece in 1889 on tobacco. It was discovered in Florida in 1900. It is the predominant pest on cotton in the United States due to the insects' increased resistance to insecticides. It resembles the greenhouse whitefly except it is slightly smaller and more yellowish, and holds its wings closer to its body. The lifecycles of the two species are quite similar. The eggs are easy to distinguish from each other, B.tabaci are yellowish green and do not change to brown. Longevity depends on temperature, at high temperatures the female lives 10-15 days, at lower temperatures they can live up to 2 months. Adults can live for an extensive time even without host plants in an empty greenhouse, however, they cannot survive temperatures below freezing. B.tabaci can transmit many viruses, including Tomato Yellow Leaf Curl Virus (TYLCV).

Natural enemies of whiteflies

There are several natural predators of whiteflies, however they tend to be very specific to particular species of whitefly hosts, therefore correct identification of the pest is critical. Any pesticide residues may adversely affect the predator population, so careful attention to integrated pest management practices are essential to the success of any biological control effort.

Encarsia formosa

Photo by Biolab, Italy

Encarsia formosa is a tiny parasitic wasp of T.vaporariorum (greenhouse whitefly). The larvae of this wasp develop inside the whitefly larvae or pupae. The parasitized greenhouse whitefly larvae are easy to recognize because they become black after about 10 days as the young wasp develops inside. Although Encarsia prefer greenhouse whitefly, they can also parasitize B. tabaci, in which case the parasitized larvae become transparent to brown in color. Adult wasps emerge from the whitefly pupa approximately 21 days after parasitization through a neat, round hole. The female adult wasp is about 0.6mm long, with a black head and thorax and a yellow abdomen. The adult wasps feed on the honeydew and body fluids of whitefly larvae. Encarsia develop faster than whiteflies, with lifecycles ranging from 3 weeks at 27C to 2 months at 14C . The population of Encarsia is almost 98% female and mating is not necessary for reproduction. The female can lay about 300 eggs in a lifetime, most of which will be more females (this is called parthengenetic reproduction).

Eretomocerus californicus The tobacco whitefly can be parasitized by Encarsia, but they are controlled better by Eretomocerus species. The Eretomocerus is another tiny parasitic wasp about the same size as the Encarsia, but without the dark head and thorax of the Encarsia species. Abundant in the Southwest U.S., Eretomocerus is reported to be well adapted to extremes of temperature and humidity, and also more resistant to pesticides than some other whitefly parasites. Females lay about 3 to 5 eggs per day, but they can also kill whitefly nymphs by repeatedly probing with their ovipositors and feeding on the haemolymph (blood) that exudes from the wounds.

Photo by Mike Rose

Verticillium lecanii

Verticillium lecanii is a common soil borne fungus which affects several different kinds of insects. It is widespread in temperate and tropical areas, but cannot infect birds, fish, mammals or plants. It was first observed on whitefly in 1915. It has a white to light yellow cotton-like appearance. The whitefly dies from infection before the fungus even becomes visible; the fungal spore germinates and begins to grow on the honeydew secretion on the whitefly body. It can either infect the insect or directly penetrate the insect. Since the fungus is not mobile and cannot seek its host, it is only effective in very high densities of white fly and repeated applications are necessary.

 

Tomato Fruit Worm

Heliothis armigera - The larva of this insect feeds on a number of plants including tomato, corn and cotton. It is sometimes called the corn earworm or the cotton bollworm. On tomato it burrows in the fruit of the tomatoes. The adult is a moth that is light yellowish in color. Control with sprays of Bacillus thurengiensis (B.t.), which is compatible with other biological control agents.

 

Photo by M. Jensen

Other pests common to hydroponic tomato production are leaf miner, tomato pinworm, cabbage looper and two-spotted spider mites. Consult your local agricultural experiment station or agricultural university for identification and control. Good sanitation is important in hydroponic tomato production, so weeds and other debris should not be allowed in and around the greenhouse as they can become a harbor in which pests can hide and multiply. A clean strip around the greenhouse, free of any plants and debris, is important.

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